Engine oil lubricates the moving parts of an engine and decreases friction and heat between the parts of the engine. Oil is made from petroleum based compounds or synthetic compounds. In either case, heat breaks down the oil. Broken down oil may tend to get watery or gummy. Accordingly, engine oil should be replaced periodically.
Replacement of the engine oil is one of the most important types of maintenance to be performed on the engine. Some engine manuals (e.g., lawnmowers or other small engines) may instruct the user to replace the engine oil every year or every season. Automotive manufacturers may recommend that engine oil be changed based on a usage distance (e.g., every 3,000 miles) or based on a period of time (e.g., every 3 months), whichever comes first.
However, the life of engine oil is not as simple as a travel distance or a usage time. Some applications of an engine, such as power generator systems, may operate in sporadic patterns that are not easily predicted by existing algorithms.